'\" t
.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pthread_setschedparam 3 2024-06-15 "Linux man-pages 6.9.1"
.SH NAME
pthread_setschedparam, pthread_getschedparam \- set/get
scheduling policy and parameters of a thread
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.P
.BI "int pthread_setschedparam(pthread_t " thread ", int " policy ,
.BI "                          const struct sched_param *" param );
.BI "int pthread_getschedparam(pthread_t " thread ", int *restrict " policy ,
.BI "                          struct sched_param *restrict " param );
.fi
.SH DESCRIPTION
The
.BR pthread_setschedparam ()
function sets the scheduling policy and parameters of the thread
.IR thread .
.P
.I policy
specifies the new scheduling policy for
.IR thread .
The supported values for
.IR policy ,
and their semantics, are described in
.BR sched (7).
.\" FIXME . pthread_setschedparam() places no restriction on the policy,
.\" but pthread_attr_setschedpolicy() restricts policy to RR/FIFO/OTHER
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=7013
.P
The structure pointed to by
.I param
specifies the new scheduling parameters for
.IR thread .
Scheduling parameters are maintained in the following structure:
.P
.in +4n
.EX
struct sched_param {
    int sched_priority;     /* Scheduling priority */
};
.EE
.in
.P
As can be seen, only one scheduling parameter is supported.
For details of the permitted ranges for scheduling priorities
in each scheduling policy, see
.BR sched (7).
.P
The
.BR pthread_getschedparam ()
function returns the scheduling policy and parameters of the thread
.IR thread ,
in the buffers pointed to by
.I policy
and
.IR param ,
respectively.
The returned priority value is that set by the most recent
.BR pthread_setschedparam (),
.BR pthread_setschedprio (3),
or
.BR pthread_create (3)
call that affected
.IR thread .
The returned priority does not reflect any temporary priority adjustments
as a result of calls to any priority inheritance or
priority ceiling functions (see, for example,
.BR pthread_mutexattr_setprioceiling (3)
and
.BR pthread_mutexattr_setprotocol (3)).
.\" FIXME . nptl/pthread_setschedparam.c has the following
.\"   /* If the thread should have higher priority because of some
.\"      PTHREAD_PRIO_PROTECT mutexes it holds, adjust the priority. */
.\" Eventually (perhaps after writing the mutexattr pages), we
.\" may want to add something on the topic to this page.
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a nonzero error number.
If
.BR pthread_setschedparam ()
fails, the scheduling policy and parameters of
.I thread
are not changed.
.SH ERRORS
Both of these functions can fail with the following error:
.TP
.B ESRCH
No thread with the ID
.I thread
could be found.
.P
.BR pthread_setschedparam ()
may additionally fail with the following errors:
.TP
.B EINVAL
.I policy
is not a recognized policy, or
.I param
does not make sense for the
.IR policy .
.TP
.B EPERM
The caller does not have appropriate privileges
to set the specified scheduling policy and parameters.
.P
POSIX.1 also documents an
.B ENOTSUP
("attempt was made to set the policy or scheduling parameters
to an unsupported value") error for
.BR pthread_setschedparam ().
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR pthread_setschedparam (),
.BR pthread_getschedparam ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
glibc 2.0
POSIX.1-2001.
.SH NOTES
For a description of the permissions required to, and the effect of,
changing a thread's scheduling policy and priority,
and details of the permitted ranges for priorities
in each scheduling policy, see
.BR sched (7).
.SH EXAMPLES
The program below demonstrates the use of
.BR pthread_setschedparam ()
and
.BR pthread_getschedparam (),
as well as the use of a number of other scheduling-related
pthreads functions.
.P
In the following run, the main thread sets its scheduling policy to
.B SCHED_FIFO
with a priority of 10,
and initializes a thread attributes object with
a scheduling policy attribute of
.B SCHED_RR
and a scheduling priority attribute of 20.
The program then sets (using
.BR pthread_attr_setinheritsched (3))
the inherit scheduler attribute of the thread attributes object to
.BR PTHREAD_EXPLICIT_SCHED ,
meaning that threads created using this attributes object should
take their scheduling attributes from the thread attributes object.
The program then creates a thread using the thread attributes object,
and that thread displays its scheduling policy and priority.
.P
.in +4n
.EX
$ \fBsu\fP      # Need privilege to set real\-time scheduling policies
Password:
# \fB./a.out \-mf10 \-ar20 \-i e\fP
Scheduler settings of main thread
    policy=SCHED_FIFO, priority=10
\&
Scheduler settings in \[aq]attr\[aq]
    policy=SCHED_RR, priority=20
    inheritsched is EXPLICIT
\&
Scheduler attributes of new thread
    policy=SCHED_RR, priority=20
.EE
.in
.P
In the above output, one can see that the scheduling policy and priority
were taken from the values specified in the thread attributes object.
.P
The next run is the same as the previous,
except that the inherit scheduler attribute is set to
.BR PTHREAD_INHERIT_SCHED ,
meaning that threads created using the thread attributes object should
ignore the scheduling attributes specified in the attributes object
and instead take their scheduling attributes from the creating thread.
.P
.in +4n
.EX
# \fB./a.out \-mf10 \-ar20 \-i i\fP
Scheduler settings of main thread
    policy=SCHED_FIFO, priority=10
\&
Scheduler settings in \[aq]attr\[aq]
    policy=SCHED_RR, priority=20
    inheritsched is INHERIT
\&
Scheduler attributes of new thread
    policy=SCHED_FIFO, priority=10
.EE
.in
.P
In the above output, one can see that the scheduling policy and priority
were taken from the creating thread,
rather than the thread attributes object.
.P
Note that if we had omitted the
.I \-i\~i
option, the output would have been the same, since
.B PTHREAD_INHERIT_SCHED
is the default for the inherit scheduler attribute.
.SS Program source
\&
.\" SRC BEGIN (pthreads_sched_test.c)
.EX
/* pthreads_sched_test.c */
\&
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
\&
#define handle_error_en(en, msg) \[rs]
        do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
\&
[[noreturn]]
static void
usage(char *prog_name, char *msg)
{
    if (msg != NULL)
        fputs(msg, stderr);
\&
    fprintf(stderr, "Usage: %s [options]\[rs]n", prog_name);
    fprintf(stderr, "Options are:\[rs]n");
#define fpe(msg) fprintf(stderr, "\[rs]t%s", msg)          /* Shorter */
    fpe("\-a<policy><prio> Set scheduling policy and priority in\[rs]n");
    fpe("                 thread attributes object\[rs]n");
    fpe("                 <policy> can be\[rs]n");
    fpe("                     f  SCHED_FIFO\[rs]n");
    fpe("                     r  SCHED_RR\[rs]n");
    fpe("                     o  SCHED_OTHER\[rs]n");
    fpe("\-A               Use default thread attributes object\[rs]n");
    fpe("\-i {e|i}         Set inherit scheduler attribute to\[rs]n");
    fpe("                 \[aq]explicit\[aq] or \[aq]inherit\[aq]\[rs]n");
    fpe("\-m<policy><prio> Set scheduling policy and priority on\[rs]n");
    fpe("                 main thread before pthread_create() call\[rs]n");
    exit(EXIT_FAILURE);
}
\&
static int
get_policy(char p, int *policy)
{
    switch (p) {
    case \[aq]f\[aq]: *policy = SCHED_FIFO;     return 1;
    case \[aq]r\[aq]: *policy = SCHED_RR;       return 1;
    case \[aq]o\[aq]: *policy = SCHED_OTHER;    return 1;
    default:  return 0;
    }
}
\&
static void
display_sched_attr(int policy, const struct sched_param *param)
{
    printf("    policy=%s, priority=%d\[rs]n",
           (policy == SCHED_FIFO)  ? "SCHED_FIFO" :
           (policy == SCHED_RR)    ? "SCHED_RR" :
           (policy == SCHED_OTHER) ? "SCHED_OTHER" :
           "???",
           param\->sched_priority);
}
\&
static void
display_thread_sched_attr(char *msg)
{
    int policy, s;
    struct sched_param param;
\&
    s = pthread_getschedparam(pthread_self(), &policy, &param);
    if (s != 0)
        handle_error_en(s, "pthread_getschedparam");
\&
    printf("%s\[rs]n", msg);
    display_sched_attr(policy, &param);
}
\&
static void *
thread_start(void *arg)
{
    display_thread_sched_attr("Scheduler attributes of new thread");
\&
    return NULL;
}
\&
int
main(int argc, char *argv[])
{
    int s, opt, inheritsched, use_null_attrib, policy;
    pthread_t thread;
    pthread_attr_t attr;
    pthread_attr_t *attrp;
    char *attr_sched_str, *main_sched_str, *inheritsched_str;
    struct sched_param param;
\&
    /* Process command\-line options. */
\&
    use_null_attrib = 0;
    attr_sched_str = NULL;
    main_sched_str = NULL;
    inheritsched_str = NULL;
\&
    while ((opt = getopt(argc, argv, "a:Ai:m:")) != \-1) {
        switch (opt) {
        case \[aq]a\[aq]: attr_sched_str = optarg;      break;
        case \[aq]A\[aq]: use_null_attrib = 1;          break;
        case \[aq]i\[aq]: inheritsched_str = optarg;    break;
        case \[aq]m\[aq]: main_sched_str = optarg;      break;
        default:  usage(argv[0], "Unrecognized option\[rs]n");
        }
    }
\&
    if (use_null_attrib
        && (inheritsched_str != NULL || attr_sched_str != NULL))
    {
        usage(argv[0], "Can\[aq]t specify \-A with \-i or \-a\[rs]n");
    }
\&
    /* Optionally set scheduling attributes of main thread,
       and display the attributes. */
\&
    if (main_sched_str != NULL) {
        if (!get_policy(main_sched_str[0], &policy))
            usage(argv[0], "Bad policy for main thread (\-m)\[rs]n");
        param.sched_priority = strtol(&main_sched_str[1], NULL, 0);
\&
        s = pthread_setschedparam(pthread_self(), policy, &param);
        if (s != 0)
            handle_error_en(s, "pthread_setschedparam");
    }
\&
    display_thread_sched_attr("Scheduler settings of main thread");
    printf("\[rs]n");
\&
    /* Initialize thread attributes object according to options. */
\&
    attrp = NULL;
\&
    if (!use_null_attrib) {
        s = pthread_attr_init(&attr);
        if (s != 0)
            handle_error_en(s, "pthread_attr_init");
        attrp = &attr;
    }
\&
    if (inheritsched_str != NULL) {
        if (inheritsched_str[0] == \[aq]e\[aq])
            inheritsched = PTHREAD_EXPLICIT_SCHED;
        else if (inheritsched_str[0] == \[aq]i\[aq])
            inheritsched = PTHREAD_INHERIT_SCHED;
        else
            usage(argv[0], "Value for \-i must be \[aq]e\[aq] or \[aq]i\[aq]\[rs]n");
\&
        s = pthread_attr_setinheritsched(&attr, inheritsched);
        if (s != 0)
            handle_error_en(s, "pthread_attr_setinheritsched");
    }
\&
    if (attr_sched_str != NULL) {
        if (!get_policy(attr_sched_str[0], &policy))
            usage(argv[0], "Bad policy for \[aq]attr\[aq] (\-a)\[rs]n");
        param.sched_priority = strtol(&attr_sched_str[1], NULL, 0);
\&
        s = pthread_attr_setschedpolicy(&attr, policy);
        if (s != 0)
            handle_error_en(s, "pthread_attr_setschedpolicy");
        s = pthread_attr_setschedparam(&attr, &param);
        if (s != 0)
            handle_error_en(s, "pthread_attr_setschedparam");
    }
\&
    /* If we initialized a thread attributes object, display
       the scheduling attributes that were set in the object. */
\&
    if (attrp != NULL) {
        s = pthread_attr_getschedparam(&attr, &param);
        if (s != 0)
            handle_error_en(s, "pthread_attr_getschedparam");
        s = pthread_attr_getschedpolicy(&attr, &policy);
        if (s != 0)
            handle_error_en(s, "pthread_attr_getschedpolicy");
\&
        printf("Scheduler settings in \[aq]attr\[aq]\[rs]n");
        display_sched_attr(policy, &param);
\&
        pthread_attr_getinheritsched(&attr, &inheritsched);
        printf("    inheritsched is %s\[rs]n",
               (inheritsched == PTHREAD_INHERIT_SCHED)  ? "INHERIT" :
               (inheritsched == PTHREAD_EXPLICIT_SCHED) ? "EXPLICIT" :
               "???");
        printf("\[rs]n");
    }
\&
    /* Create a thread that will display its scheduling attributes. */
\&
    s = pthread_create(&thread, attrp, &thread_start, NULL);
    if (s != 0)
        handle_error_en(s, "pthread_create");
\&
    /* Destroy unneeded thread attributes object. */
\&
    if (!use_null_attrib) {
      s = pthread_attr_destroy(&attr);
      if (s != 0)
          handle_error_en(s, "pthread_attr_destroy");
    }
\&
    s = pthread_join(thread, NULL);
    if (s != 0)
        handle_error_en(s, "pthread_join");
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.ad l
.nh
.BR getrlimit (2),
.BR sched_get_priority_min (2),
.BR pthread_attr_init (3),
.BR pthread_attr_setinheritsched (3),
.BR pthread_attr_setschedparam (3),
.BR pthread_attr_setschedpolicy (3),
.BR pthread_create (3),
.BR pthread_self (3),
.BR pthread_setschedprio (3),
.BR pthreads (7),
.BR sched (7)
